The present invention relates to network analysis, and more particularly to a network analyzer that uses time sequenced measurements to make RF measurements.
In all network analyzers two RF measurements are required to determine the magnitude and phase of a reflection coefficient. As shown in FIG. 1, which corresponds to FIG. 7 in U.S. Pat. No. 5,642,039 (Bradley et al), a sample of the signal that is applied to a return loss bridge is used to establish its magnitude and phase. It is called the reference signal. Also a return or test signal output from the return loss bridge is measured to determine this signal""s magnitude and phase. It is called the test signal. The reflection coefficient of the device under test is then determined as the ratio of the test signal rationated or divided by the reference signal. In this way the arbitrary amplitude and phase of the signal source is removed from the measurement results. Bradley et al teach that the two measurements are made simultaneously.
An advantage of making the measurements simultaneously is that it makes the effects of phase noise in the measurements much smaller. The disadvantage is that the effects of small leakage signals traveling between the two measurement processes due to imperfect shielding or imperfect circuit elements may cause measurement error in one or both of the measurements. The direct leakage problem is driven by two issues: (1) lack of appropriate shielding; and (2) leakage of source signal from the mixer input to its local oscillator terminals, backward along the local oscillator path to a common point with the transmission mixer local oscillator signal and hence forward to the transmission mixer along its local oscillator path.
What is desired is a network analyzer that determines RF measurements for a device under test while minimizing signal source leakage impact upon the measurements.
Accordingly the present invention provides a network analyzer using time sequenced measurements that isolates respective channel mixers from a signal source to ease problems caused by direct leakage from a signal source. The signal source is set to a desired measurement frequency and a local oscillator is set to a desired frequency so that, when mixed, a desired IF frequency is achieved. The signal source output is applied to an impedance test terminal, to which also is coupled a device under test, and to a reference channel mixer. A return loss signal from the impedance test terminal is applied to a test channel mixer. The LO output signal is applied in a time sequence in any order via an LO switch first to one channel mixer and then to the other channel mixer. An output switch couples the respective mixers to a processor in the same time sequence to provide sequential measurements of magnitude and phase for each channel from which reflection coefficients are derived for the device under test. Likewise gain/loss measurements may be made by coupling the device under test between the impedance test port and a transmission test port, the transmission test port being coupled to a transmission channel mixer to which also is input the LO output via the LO switch in the time sequence. The output of the transmission channel mixer is coupled via the output switch to the processor to obtain the gain/loss measurements.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.